Semiconductor memory devices are commonly used to store information (either temporarily or permanently) in a number of applications; particularly, in a non-volatile memory device the information is preserved even when a power supply is off. Typically, the memory device includes a matrix of memory cells that are arranged in a plurality of rows (connected to corresponding word lines) and in a plurality of columns (connected to corresponding bit lines).
For example, flash memory devices are a particular type of non-volatile memory device, in which each memory cell is formed by a floating gate MOSFET transistor. Each memory cell has a threshold voltage (which depends on the electric charge stored in the corresponding floating gate), which can be programmed to different levels representing corresponding logical values. Particularly, in a multi-level flash memory device each memory cell can take more than two levels (and then store a plurality of bits).
In order to retrieve and/or store information, the flash memory device includes a decoding system that is adapted to decode an addressing code identifying a group of memory cells. In particular, the decoding system includes a row selector for selecting a corresponding word line and a column selector for selecting a corresponding set of bit lines. These selectors operate with logical signals at low voltages, of the order of a supply voltage of the flash memory device (such as 3V); for example, the logical signals can take two values equal to a reference voltage (0) or to the supply voltage (1).
However, the decoding system often must be able to apply operative voltages of high value to the selected memory cells (during program and erase operations). These voltages (for example, ranging from −9V to 9V) are higher than the supply voltage (in absolute value). For example, in single supply voltage memory devices, the high voltages are generated inside the flash memory device from the supply voltage, by means of suitable circuits (such as, charge pumps).
For this purpose, the decoding system includes level shifters, which are adapted to convert the logical signals from the selectors into the high voltages necessary during the program and erase operations; for example, during a program operation the level shifters shift the supply voltage used for selecting the desired word line to a program voltage to be applied thereto (such as, 9V).
Therefore, the selectors can be implemented with low voltage components that are designed in such a way to be able to sustain (between their terminals) voltage differences that are limited (in absolute value) by the supply voltage. Indeed, the low voltages that are experienced by those components allow their correct functioning, without causing breaking thereof. For example, those components are low-voltage MOSFET transistors, which are designed in such a way to avoid the occurrence of gate oxide breakdown or undesired junction breakdown when low voltage differences are applied to their terminals (for example, between the gate and source terminals).
Conversely, the level shifters typically must include high-voltage components that are designed in such a way to be able to sustain (between their terminals) voltage differences that are higher than the supply voltage (up to 9V in the case at issue). For example, those components are high-voltage MOSFET transistors, which are designed in such a way to avoid the occurrence of gate oxide breakdown or undesired junction breakdown even when high voltages are applied to their terminals.
The high-voltage transistors have a gate oxide layer thicker than that used for the low voltage transistors. Indeed, the thicker the gate oxide layer the higher the voltage sustained at their terminals without undesired breakdown. Since the high-voltage transistors typically occupy more silicon area compared to the low-voltage transistors, the level shifters may waste a significant area of a chip wherein the flash memory device is integrated.
This problem is more evident with the increment of the number of word lines included in the flash memory device.
Moreover, the use of both low- and high-voltage transistors often increases the number of processing steps and masks (for example, for differentiating the oxide thickness of the high- and low-voltage transistors); this may have a detrimental impact on the manufacturing process of the flash memory device.